Treatment for produced and flowback waters from wells

ABSTRACT

Produced and flowback fluids, and other fluids emanating from oil, mining, and mineral extraction operations, are treated to remove heavy metals by introducing an oxidizing agent and passing the fluid through an electrocoagulator. A cavitation device is used to intensify the oxidation reactions. Coalesced bodies made in the electrocoagulator, including heavy metals such as iron rendered insoluble by elevation of their oxidation states, are separated from the fluids so they may be reused.

RELATED APPLICATIONS

This application claims the full benefit of Provisional Applications 61/201,105 and 61/200,968, both filed Dec. 5, 2008.

TECHNICAL FIELD

An oxidizing agent is introduced to produced and recirculated (“flowback”) aqueous fluids from wells before processing in an electrocoagulator. Hypochlorite may be generated on site from a portion of the treatment fluid to be used as the oxidizing agent for introduction to the main stream before passing to the electrocoagulator. Efficiency of the process is enhanced by passing the fluid through a cavitation device, which improves mixing, accelerates oxidation of metals, kills bacteria, and promotes precipitation and other reactions, thereby enabling the electrocoagulator to handle better the large quantities of fluid presented in the hydrocarbon recovery process. The process may also be used to reclaim usable water from acid mine drainage.

BACKGROUND OF THE INVENTION

In the drilling of wells and the recovery of hydrocarbons from them, aqueous drilling fluids and other aqueous fluids are circulated to the bottom of the well to recover the drillings, and to treat the well for various purposes. Returning to the surface, the fluid commonly brings with it “produced fluid,” such as connate fluid and other materials picked up from the well. Such additional fluids may or may not originate with the drilling rig. I use the term “flowback fluid” to mean the original drilling fluid, completion fluid, workover or other fluid injected into the well by the operator and returned to the surface. My invention is applicable to either type of fluid if it is delivered to the earth's surface separately, but most frequently the produced fluid becomes mixed with the flowback fluid.

Heavy metals, typically and especially iron, scale-forming materials such as calcium, solid particles such as bits of shale or rock (sediment or silt), lighter solids, and oil can be detrimental to the reuse of produced and flowback fluids. The calcium is generally dissolved from the earth formation.

The fluid will commonly contain a high concentration of alkali metal halides, mainly sodium chloride, but also potassium chloride or bromide, and sodium bromide. Frequently these materials are desirable for reuse; the heavier ones likely were placed in the fluid to adjust its specific gravity to provide buoyancy for the drill cuttings.

Thus the need is for an efficient system for removing the contaminants in produced and flowback waters in the hydrocarbon recovery industry. Brine-forming components are not incompatible with reuse and in fact are usually beneficial; thus the objective is to remove the heavy metals, the solids, and the scale-forming materials so that the fluid can be reused rather than having to prepare additional treatment fluids while disposing of the used fluid without causing environmental problems.

SUMMARY OF THE INVENTION

My invention combines the use of an electrocoagulator with a cavitation device for treating aqueous fluids emanating from the earth in an oilfield, mining, or mineral extraction operation, such as produced and flowback fluids, which may arrive at the treatment site in large quantities and high flow rates. My invention further includes generating hypochlorite on site from the chloride-containing produced and flowback fluid, and using the hypochlorite so generated to oxidize heavy metsls and other materials in the electrocoagulator, further enhanced by the cavitation device. After the fluid is treated by my process, it may be recycled or used in another operation or process involving the use of water in the earth.

My invention is particularly adapted to treat the wide variety of such mixed fluids encountered in hydrocarbon production. It may also be used to treat acid mine drainage, which has long been a problem for coal and other types of mines The ability to process such diverse fluids so the water can be reused has excellent environmental and economic benefits. When I generate the oxidizing agent sodium (or other alkali metal or acidic) hypochlorite from the processed fluid, it must contain a minimum amount of chloride ion. Therefore I call the subject of my treatment “chloride-containing produced and flowback fluid.”

The principle of the electrocoagulator is well known—a plurality of electrodes, usually steel or aluminum, are placed in a vessel suitable for handling electrolysis, and a direct current is applied to the solution or dilute slurry within it. Usually the electrodes are disposed as alternate anodes and cathodes, As the aqueous fluid flows through, the current causes ionic charges to be applied to the particles, colloids, heavy metal components, and the like (I sometimes refer to these materials as “coagulant bodies”), which facilitates oxidation, precipitation, flocculation, and other events tending to cause a separation of the contaminants from the aqueous carrier. As is known in the art of electrolysis, a certain level of electrolyte concentration is necessary for optimum operation of an electrolytic cell, and a similar principle is true of the electrocoagulator. An electrocoagulator is typically followed by one or more devices for collecting precipitants and the like; such devices include settling vessels, filters, and further chemical treatment vessels. I have found, however, that the electrocoagulator is generally not able to prepare produced and flowback fluids for reuse by itself. Passing the fluid from the electrocoagulator to a cavitation device prior to any collection devices greatly enhances the treatment results. Filtration is also helpful before treatment in the electrocoagulator, to minimize fouling. In my flow scheme, the electrocoagulator may be preceded by filtration, addition of a flocculant or coagulant, or a pH adjustment.

An electrocoagulation device described in 1982 U.S. Pat. No. 4,329,211 to Plantes et al is said to have accomplished the oxidation of iron to form insoluble compounds in waste water, rendering them more amenable to agglomeration and removal. The electrocoagulator of the '211 patent is said to be an improvement on an earlier design. This patent, U.S. Pat. No. 4,329,211 to Plantes et al is hereby specifically incorporated herein, in its entirety, by reference.

U.S. Pat. No. 6,488,835 describes an electrocoagulator and method of electrocoagulation wherein the electrode plates are consumed (sacrificial), providing a procedure for replacing them. This U.S. Pat. No. 6,488,835 to Powell is also specifically incorporated herein, in its entirety, by reference.

In Stephenson et al U.S. Pat. No. 6,346,197, a basic electrocoagulator unit is illustrated relatively simply in FIGS. 2 and 3. Conductive plates are alternatingly connected to oppositely charged electrodes, providing an equal amount of anode and cathode conductive plates. In this construction, the plates are large in area and few in number, permitting lower pressure and voltage drops. They are preferably aluminum but may be made of various other metals and alloys. They are arranged to define a serpentine flow path. This U.S. Pat. No. 6,346,197 to Stephenson et al is also specifically incorporated herein by reference in its entirety.

In the U.S. Pat. No. 6,719,894 to Gavrel et al, an electrocoagulator vessel is described including parallel electrolytic plates therein; fluid in the vessel is subject to pressure manipulation to enhance solids removal. This patent U.S. Pat. No. 6,719,894 to Gavrel et al is also specifically incorporated herein by reference in its entirety.

FIG. 3 in particular of Robinson U.S. Pat. No. 6,800,206 also presents the basic principles of an electrocoagulator. The anodes are sacrificial, providing ions to assist In treating the fluid passing through, and the number of anodes actually passing current may be controlled as a function of the conductivity of the fluid. This U.S. Pat. No. 6,800,206 to Robinson is also hereby specifically incorporated herein by reference in its entirety.

A tubular configuration is shown for an electrocoagulator in FIG. 2 of Bradley U.S. Pat. No. 6,960,301. As mentioned therein (col. 6, lines 56-60) factors influencing the rate of coagulation include the residence time of the fluid in the device, the applied current and voltage, turbulent flow characteristics of the fluid, temperature, electrode surface area, and concentrations of various contaminants in the fluid. As with other electrocoagulators, the Bradley device is said to be effective in removing bacteria as well as metals from arsenic to zinc, organic solids and anionic species amenable to coagulation: column 5, lines 9-22. The Bradley U.S. Pat. No. 6,960,301 is also incorporated herein specifically in its entirety.

The above references to various designs and types of electrocoagulators are not intended to be limiting, but rather to illustrate that any workable design for an electrocoagulator may be used in my invention; any of the above may be used as well as any commercial electrocoagulator on the market having an appropriate capacity and the abilities described herein.

Electrocoagulators are particularly well adapted to oxidize heavy metals such as iron. When an oxidizing agent is introduced ahead of the electrocoagulator, the oxidizing reaction is accelerated in the electrocoagulator, but the oxidation reaction will generally require a residence time for completion which may not be practical for the electrocoagulator alone. This appears to be the reason it has not found use in oilfield fluid reclamation.

The electrocoagulator in my invention is adapted to handle the high flow rates containing a variety of contaminants. Generally it should be able to handle a flow rate of 100 to 400 gallons per minute. For a typical flow rate of 200 gallons per minute (gpm), a generator or power source on site should be able to deliver 480 V and 400 amps. To prevent scale build up and to evenly wear the plates, the charge should be alternated every few minutes. When the phase changes, there is a surge, thus the 400 amp is needed. Steady state treatment of 200 gpm normally requires 200 amperes. I do not intend to be limited to electrocoagulators having the capabilities or specifications just mentioned; they can of course be somewhat smaller and considerably larger, depending on the expected flow rates and other conditions; the principle of operation remains substantially the same.

An oxidizing agent is added to the fluid prior to its introduction to the electrocoagulator. While there may be air, and hence oxygen, entering the fluid through pumps, valves and the like as well as the electrocoagulator itself, the oxygen not deliberately dissolved in the fluid will not be enough to oxidize the iron frequently found in the fluids I treat and elevate its valence state to achieve an insoluble form. Moreover, other metals will commonly be found in the fluid, and these may consume some of the oxygen provided by aeration. In addition, an added oxidizing agent is desirable to kill or coagulate bacteria. It should be kept in mind that it is an important object of the invention to prepare produced and flowback fluids for recirculation to a well, to minimize the usage of new water; treatment of acid mine drainage also permits practical use of an otherwise contaminated water in a way that first removes many of its worst ingredients.

After treatment in the electrocoagulator, the fluid may be sent to the cavitation device, which functions as a simultaneous heater and intimate mixer. It is particularly good at promoting the oxidation reaction of heavy metals present in the fluid. The oxidation of heavy metals such as iron produces heavy metal oxides which will settle out under the proper conditions, given the high volume and flow rates of the fluid, but it is generally impossible to achieve the residence time in the EC necessary to complete the oxidation reaction. In the cavitation device, the increased temperature and the intimate mixing assure completion of the reaction.

Almost all used clear completion fluids, and also many drilling fluids, and thus usually the produced and flowback fluids I treat, contain iron, which has historically been extremely difficult to remove in the process of cleaning and preserving the fluids for reuse. Iron is generally in the form of FeO, which is soluble in the low pH common in completion fluids. Dissolved iron in the form of FeO cannot be filtered unless it is oxidized to a higher oxidative state. Raising the pH should be considered only while recognizing the counterproductive possibility of precipitating out some useful zinc or even bromide salts. The fluid frequently incorporates dissolved oxygen from the air with normal pumping and handling; this may convert some of the iron to Fe₂O₃ in the form of a 0.5 micron colloidal suspension, but the quantity of oxygen dissolved in this manner is seldom anywhere near enough. Iron is a pervasive component of acid mine drainage fluids and can be removed by my invention in a manner similar to that by which I treat produced and flowback fluids.

As indicated above, a small amount of oxygen or air can always be expected to be dissolved in the treated fluid, and this oxygen is available to oxidize heavy metals under the appropriate conditions. Oxidation is enhanced by the injection of an oxidizing agent ahead of the electrocoagulator. Generally, up to about 100 ppm O₂ or equivalent will be used, but the amount will depend on operator's knowledge of iron content and other prevelant conditions. Ozone and other convenient oxidizing agents, such as hydrogen peroxide, may also be used. The performance of hydrogen peroxide may be enhanced by the addition of ferric chloride. And, the oxidation reaction may be enhanced by the use of a cavitation device between the oxidizer addition and the electrocoagulator.

Where the treated fluid has a significant chlorine content, which is common in produced and flowback fluids where, for example, a drilling fluid contains high concentrations of alkali metal chlorides) alkali metal hypochlorites may be made in situ by a hypochlorite electrolytic cell or other device. The manufacture of alkali metal hypochlorite, particularly sodium hypochlorite, is also well known. See, for example, Langeland et al U.S. Pat. No. 4,783,246 showing the use of flat, platelike bipolar electrodes in an electrolytic cell. See also Bennett et al U.S. Pat. No. 3,849,281. The constructions and methods of these patents are illustrative of the basic art of using electrolysis to make sodium or potassium hypochlorite (cesium may be present in some oilfield brines as well), but I do not intend to be limited to the temperatures, concentrations of chloride, and pH ranges recited in them. Nevertheless, both the Langeland et al patent U.S. Pat. No. 4,783,246 and the Bennet et al U.S. Pat. No. 3,849,281 are hereby specifically incorporated herein by reference in their entireties. Any commercially available hypochlorite generator having a capacity suitable for the flow rates and volumes necessitated by the flow and volume of the produced and flowback fluids treated herein may be used.

Preferably the cavitation device is one manufactured and sold by Hydro Dynamics, Inc., of Rome, Ga., most preferably the device described in U.S. Pat. Nos. 5,385,298, 5,957,122 6,627,784 and particularly 5,188,090, all of which are incorporated herein by reference in their entireties.

Definition: I use the term “cavitation device” to mean and include any device which will cause bubbles or pockets of partial vacuum to form within the liquid it processes. The bubbles or pockets of partial vacuum have also been described as areas within the liquid which have reached the vapor pressure of the liquid. The turbulence and/or impact, which may be called a shock wave, caused by the implosion imparts thermal energy to the liquid. The bubbles or pockets of partial vacuum are typically created by flowing the liquid through narrow passages which present side depressions, cavities, pockets, apertures, or dead-end holes to the flowing liquid; hence the term “cavitation effect” is frequently applied, and devices known as “cavitation pumps” or “cavitation regenerators” are included in my definition. Steam generated in the cavitation device can be separated from the remaining, now concentrated, water and/or other liquid which frequently will include significant quantities of solids small enough to pass through the reactor. Cavitation devices can be used to heat fluids, but in my invention I use them as excellent intimate mixing devices. I therefore rely mainly on the shearing stress and turbulence imparted to the liquid as it passes through the narrow passages between the rotor and the concentric housing surface. The term “cavitation device” includes not only all the devices described in the above itemized patents U.S. Pat. No. 5,385,298, 5,957,122 6,627,784 and 5,188,090 but also any of the devices described by Sajewski in U.S. Pat. Nos. 5,183,513, 5,184,576, and 5,239,948, Wyszomirski in U.S. Pat. No. 3,198,191, Selivanov in U.S. Pat. No. 6,016,798, Thoma in U.S. Pat. Nos. 7,089,886, 6,976,486, 6,959,669, 6,910,448, and 6,823,820, Crosta et al in U.S. Pat. No. 6,595,759, Giebeler et al in U.S. Pat. Nos. 5,931,153 and 6,164,274, Huffman in U.S. Pat. No. 5,419,306, Archibald et al in U.S. Pat. No. 6,596,178 and other similar devices which employ a shearing effect between two close surfaces, at least one of which is moving, such as a rotor, and at least one of which will normally have cavities of various designs in its surface as explained above, but for the intimate mixing purposes of my invention, a cavitation effect is not essential and therefore the term cavitation device as used herein should be read to include other devices which will generate shear between two close surfaces, one of which is moving.

An additional benefit for my invention from the presence of copious amounts of chlorides is that I can generate hypochlorite directly from the fluid on site and use it to oxidize heavy metals and other materials in the fluid.

The cavitation device also facilitates the removal of calcium and other scale-forming materials by intimately mixing the fluid with sodium bicarbonate injected between the electrocoagulator and the cavitation device, thus assuring completion of the reaction to form calcium carbonate, which can be removed in later separation steps. The efficiency of any inorganic or organic coagulant or flucculant, including polymers, will be enhanced by the intimate mixing afforded by the cavitation device.

Zinc, aluminum, nickel, manganese, magnesium, cadmium and copper may be found in acid mine drainage as well as iron. Some of these metal forms are toxic, and significant amounts of sulfuric acid are typical of acid mine drainage compositions. Accordingly acid mine drainage is a challenging problem for remediation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a and 1 b illustrate a cavitation device useful in my invention.

FIG. 2 shows the disposition of the electrodes in an electrocoagulator useful in my invention.

FIG. 3 is a flow sheet showing the placement of the hypochlorite generator, the EC, and the cavitation device.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 a and 1 b show two slightly different variations, and views, of a cavitation device.

FIGS. 1 a and 1 b are taken from FIGS. 1 and 2 of Griggs U.S. Pat. No. 5,188,090, which is incorporated herein by reference along with related US patents U.S. Pat. Nos. 5,385,298, 5,957,122, and 6,627,784.

A housing 10 in FIGS. 1 a and 1 b encloses cylindrical rotor 11 leaving only a small clearance 12 around its curved surface and clearance 13 at the ends. The rotor 11 is mounted on a shaft 14 turned by motor 15. Cavities 17 are drilled or otherwise cut into the surface of rotor 11. As explained in the Griggs patents, other irregularities, such as shallow lips around the cavities 17, may be placed on the surface of the rotor 11. Some of the cavities 17 may be drilled at an angle other than perpendicular to the surface of rotor 11—for example, at a 15 degree angle. Liquid (fluid)—in the case of the present invention, the produced and/or backflow fluid,—is introduced through port 16 under pressure and enters clearances 13 and 12. As the fluid passes from port 16 to clearance 13 to clearance 12 and out exit 18 while the rotor 11 is turning, areas of vacuum are generated within the fluid from its own turbulence, expansion and compression. As explained at column 2 lines 61 et seq in the U.S. Pat. No. 5,188,090 patent, “(T)he depth, diameter and orientation of (the cavities) may be adjusted in dimension to optimize efficiency and effectiveness of (the cavitation device) for heating various fluids, and to optimize operation, efficiency, and effectiveness . . . with respect to particular fluid temperatures, pressures and flow rates, as they relate to rotational speed of (the rotor 11).” Smaller or larger clearances may be provided (col. 3, lines 9-14) to adjust shear. Also the interior surface of the housing 10 may be smooth with no irregularities or may be serrated, feature holes or bores or other irregularities as desired to increase efficiency and effectiveness for particular fluids, flow rates and rotational speeds of the rotor 11. (col. 3, lines 23-29) Rotational velocity may be on the order of 5000 rpm (col 4 line 13). The diameter of the exhaust ports 18 may be varied also depending on the fluid treated. Note that the position of exit port 18 is somewhat different in FIGS. 1 a and 1 b; likewise the position of entrance port 16 differs in the two versions and may also be varied to achieve different effects in the flow pattern within the cavitation device.

Another variation which can lend versatility to the cavitation device is to design the opposing surfaces of housing 10 and rotor 11 to be somewhat conical, and to provide a means for adjusting the position of the rotor within the housing so as to increase or decrease the width of the clearance 12 to adjust shear. This can allow for different sizes of solids present in the fluid, to reduce the shearing effect if desired (by increasing the width of clearance 12), to vary the velocity of the rotor as a function of the fluid's viscosity, or for any other reason.

Operation of the cavitation device is as follows. A shearing stress is created in the solution as it passes into the narrow clearance 12 between the rotor 11 and the housing 10. The solution quickly encounters the cavities 17 in the rotor 11, and tends to fill the cavities, but the centrifugal force of the rotation tends to throw the liquid back out of the cavity. Small bubbles, some of them microscopic, are formed. Where no gas is present, the small bubbles are imploded.

FIG. 2 shows the disposition of the electrodes in an electrocoagulator useful in my invention.

FIG. 2 depicts the disposition of the electrodes in an electrocoagulator of a type suitable for my invention. The vessel or housing is not shown; nor is the fluid to be treated. The vessel or housing should have a suitable entrance and a suitable exit for the fluid, and should be able to accommodate the rather high flow rates contemplated by the process. Parallel electrodes 30 and 31 are desirably, but need not be, completely submerged in the fluid, and are given alternate positive and negative functions under direct current Here it is seen that the electrodes 30 are positively charged and electrodes 31 are negatively charged. When made of steel, iron and other metals, the anodes will tend to erode as sacrificial anodes. In order to balance this effect, the current is reversed periodically, typically every few minutes; reversing the current will also minimize scale formation on the electrodes.

Desirably the gaps between the electrodes will be adjustable so the operator can obtain optimum benefit for different compositions of fluid. As indicated above, the power requirements (current) will surge each time the phase is changed, and accordingly assumptions about steady state may not suffice when designing the electrocoagulator. The operator will wish to avoid conditions likely to generate chlorine gas.

FIG. 3 is a flow sheet showing the placement of the hypochlorite generator, the electrocoagulator and the cavitation device. Produced water and/or flowback water (or other fluid as described herein, including acid mine drainage) is moved through conduit 41 from tank 40 or other source such as a pipe into the electrocoagulator 42 (“EC unit”). A portion of the fluid is directed in line 48 to hypochlorite generator 47, which is an electrolytic cell adapted to generate hypochlorite from the chloride-containing fluid. The hypochlorite generated in the hypochlorite generator 47 is injected through line 50 into conduit 41 upstream of the electrocoagulator 42. In the EC unit 42, the fluid is subjected to a direct current as described with respect to FIG. 2, bringing about oxidation and other reactions in the components of the fluid. The fluid is then passed through conduit 43 to the cavitation device 44 where it is subjected to shear, moderate temperature elevation, and intimate mixing, facilitating efficient contact of the reactants on the molecular level. I call this intensifying the reactions. Various copolymers and/or anionic or cationic polymers can be injected into the fluid in conduit 43 from feeders 51, and this is an excellent point at which to inject a bicarbonate compound to react with calcium and other scale-forming polyvalent metals, as indicated by bicarbonate source 45. The cavitation device will enhance the precipitation of calcium carbonate as well as enhance the oxidation of FeO to a higher oxidative state such as Fe₂O₃, which is more readily coagulated and more readily filterable.

The fluid proceeds in line 46 as indicated in FIG. 3, to a solids/liquid separation section, where polymeric flocculants and/or inorganic coagulants may enhance the separation of solids and other materials by filtration, settling or other means. Possible separation devices indicated by reference numbers 52 and 53 include lamella gravity settlers, tube settlers, and various filters. Scale inhibitors and biocides may be added to the cleaned fluid from source 54 prior to use in a well or for other purposes.

Hypochlorite generator 47 is basically an electrolytic cell having one or more anodes and one or more cathodes, operated under conditions to generate hypochlorite from the chloride-containing fluid. After the process is begun, the hypochlorite generator 47 may be fed additionally or alternatively by a slip stream from the fluid at a point downstream from the cavitation device, such as through line 49, in order to minimize fouling in the hypochlorite generator 47. Any suitable electrodes may be used in the hypochlorite generator. Where the chloride content of the fluid is insufficient, simple injection of an oxidizing agent such as oxygen, air, or hydrogen peroxide may be substituted. It is to be understood that oxidizing agents such as hydrogen peroxide and alkali metal hypochlorite may be expected to perform very well as bactericides, and the operator should keep this in mind particularly where sulfate-reducing bacteria are present.

A cavitation device may be interposed between oxidizing agent introduction and the electrocoagulator; the cavitation device will intensify the oxidation reaction by increasing the temperature of the fluid and intimately and violently mixing the contents of the fluid prior to entering the electrocoagulator. In this case, the operator may decide not to use a device positioned as cavitation device 44, but the process would benefit from cavitation devices at both locations. Use of a single cavitation device at either location is within my invention.

Thus it is seen that my invention includes a method of treating an aqueous fluid emanating from the earth in an oilfield, mining, or mineral extraction operation to remove soluble heavy metals contained therein in preparation for reuse comprising (a) introducing an oxidizing agent to the fluid to elevate the oxidation state of at least some of the soluble heavy metals, thereby converting the at least some soluble heavy metals to insoluble heavy metals, (b) passing the fluid through a cavitation device to mix and heat the fluid containing the oxidizing agent, thereby enhancing the rate of conversion of the soluble heavy metals to insoluble heavy metals, (c) passing the fluid into an electrocoagulator to coalesce at least some of the insoluble heavy metals into coagulant bodies, and (d) separating the coagulant bodies from the fluid.

My invention also includes a method of treating an aqueous fluid emanating from the earth in an oilfield, mining, or mineral extraction operation to remove soluble heavy metals contained therein and to prepare the fluid for reuse comprising (a) introducing an oxidizing agent to the fluid to elevate the oxidation state of at least some of the soluble heavy metals, thereby converting the at least some soluble heavy metals to insoluble heavy metals, (b) passing the fluid into an electrocoagulator to coalesce at least some of the insoluble heavy metals into coagulant bodies, (c) passing the fluid through a cavitation device to mix and heat the fluid, thereby enhancing the rate of conversion of the soluble heavy metals to insoluble heavy metals, thereby forming additional coagulant bodies, and (d) separating the coagulant bodies from said

And, my invention includes a method of treating aqueous oilfield chloride-containing produced and flowback fluid containing heavy metal components comprising (a) electrolytically generating hypochlorite in a side stream or portion of the fluid, (b) injecting the side stream or portion containing hypochlorite into the fluid, (c) passing the fluid containing the hypochlorite through an electrocoagulator, thereby at least partially oxidizing the heavy metal components, and (d) separating the at least partially oxidized heavy metal components from said fluid. 

1. Method of treating an aqueous fluid emanating from the earth in an oilfield, mining, or mineral extraction operation to remove soluble heavy metals contained therein in preparation for reuse comprising (a) introducing an oxidizing agent to said fluid to elevate the oxidation state of at least some of said soluble heavy metals, thereby converting said at least some soluble heavy metals to insoluble heavy metals, (b) passing said fluid through a cavitation device to mix and heat said fluid containing said oxidizing agent, thereby enhancing the rate of conversion of said soluble heavy metals to insoluble heavy metals, (c) passing said fluid into an electrocoagulator to coalesce at least some of said insoluble heavy metals into coagulant bodies, and (d) separating said coagulant bodies from said fluid.
 2. Method of claim 1 wherein step (d) is performed at least partly by a filter.
 3. Method of claim 1 wherein step (d) is accomplished at least partly by adding a coagulant or flocculating agent, followed by settling.
 4. Method of claim 1 including, between step (c) and step (d), passing said fluid through a second cavitation device to further mix and heat said fluid, thereby causing further coalescence of said insoluble heavy metals.
 5. Method of claim 1 wherein said aqueous fluid comprises acid mine drainage.
 6. Method of claim 1 wherein said aqueous fluid comprises produced and flowback fluid in an oilfield operation, followed by the step of reusing said fluid in an oilfield operation.
 7. Method of claim 1 wherein said aqueous fluid comprises produced and flowback fluid contain alkali metal chlorides, and including the steps of (i) generating alkali metal hypochlorite by electrolysis of said fluid and (ii) utilizing said alkali metal hypochlorite as the oxidizing agent of step (a).
 8. Method of treating an aqueous fluid emanating from the earth in an oilfield, mining, or mineral extraction operation to remove soluble heavy metals contained therein and to prepare said fluid for reuse comprising (a) introducing an oxidizing agent to said fluid to elevate the oxidation state of at least some of said soluble heavy metals, thereby converting said at least some soluble heavy metals to insoluble heavy metals, (b) passing said fluid into an electrocoagulator to coalesce at least some of said insoluble heavy metals into coagulant bodies, (c) passing said fluid through a cavitation device to mix and heat said fluid, thereby enhancing the rate of conversion of said soluble heavy metals to insoluble heavy metals, thereby forming additional coagulant bodies, and (d) separating said coagulant bodies from said fluid.
 9. Method of claim 8 wherein step (d) is performed at least partly by a filter.
 10. Method of claim 8 wherein step (d) at least partly comprises flocculation and settling.
 11. Method of claim 8 wherein said aqueous fluid comprises acid mine drainage. 12 Method of claim 8 wherein said aqueous fluid comprises produced and flowback fluid in an oilfield operation.
 13. Method of claim 12 wherein said produced and flowback fluid contain alkali metal chlorides, and including the steps of (i) generating alkali metal hypochlorite by electrolysis of said fluid and (ii) utilizing said alkali metal hypochlorite as the oxidizing agent of step (a).
 14. Method of treating aqueous oilfield chloride-containing produced and flowback fluid containing heavy metal components comprising (a) electrolytically generating hypochlorite in a side stream or portion of said fluid, (b) injecting said side stream or portion containing hypochlorite into said fluid, (c) passing said fluid containing said hypochlorite through an electrocoagulator, thereby at least partially oxidizing said heavy metal components, and (d) separating said at least partially oxidized heavy metal components from said fluid.
 15. Method of claim 14 wherein step (d) comprises filtering said fluid.
 16. Method of claim 14 wherein step (d) comprises collecting said heavy metal components in a settling tank.
 17. Method of claim 14 wherein said electrocoagulator is capable of handling at least 200 gallons per minute of fluid with at least 200 amperes current.
 18. Method of claim 14 wherein said electrocoagulator comprises sacrificial anodes.
 19. Method of claim 14 followed by reusing said fluid in an oilfield operation.
 20. Method of claim 14 including, between step (b) and step (d), passing said fluid through a cavitation device. 